The official declined to provide more details on the military flights, except to say that glasslike deposits were found inside the planes’ engines after they patrolled over European airspace.

Last week, two Finnish Air Force F-18 fighter-bombers suffered similar damage while flying through the ash plume that has paralyzed air traffic over much of Europe. Both landed safely, but their jet engines will require expensive overhauls.

Volcanic ash tends to stick to a jet engine’s interior parts, such as the turbines, where it melts to form a glassy coating. This restricts air flow and heats up the engine, which can lead to engine failure.